The present invention relates to metrology and more particularly to metrology which employs interferometric optical techniques. The invention more particularly relates to an improved Michelson optical fiber interferometer and its application in particular in the measurement of temperatures in flows for example within turbo-machines.
Metrology which employs interferometry is well known. This technique, which employs interferences produced between a reference wave and a measured wave which is affected by the phenomenon to be studied, is characterized by its very high sensitivity.
Ever since optical fibers have been of utility for technical purposes, it has already been proposed to use them for constructing for example Michelson or Mach-Zehnder interferometers. The use of optical fibers for interferometers has permitted the construction of apparatus which are insensitive to electromagnetic fields, have low propagation losses, are practically immune to chemical attack and moreover have compact dimensions and low mass.
As is known, interferometry permits translating an optical difference of operation between a reference wave and a measured wave into a phase difference. This phase difference .phi. is equal to ##EQU1## where: .lambda. is the wavelength of the radiation employed,
.DELTA. is the optical difference of operation with .DELTA.=nl, PA1 .eta. is the index of refraction of the medium, and PA1 l is the geometrical difference between the optical paths of the reference and measured waves in their respective arms.
In the case where there are employed a reference arm (r) and a measurement arm (m), there is strictly obtained EQU .DELTA.=n.sub.r l.sub.r -n.sub.m l.sub.m.
Any phenomenon which has an action on the geometrical difference 1 of the two optical paths and/or on the index of refraction n of the measurement arm or on the index of the reference arm, therefore results in a variation in the optical difference of operation. This property is employed for revealing physical phenomena and in particular temperature variations.
In order to guard against disturbances which may affect the reference arm and the measurement arm outside the measuring zone, i.e. the optical head, and thus render the results inaccurate, it has already been proposed to employ, for constituting the arms, optical fibers which are associated with each other by means of optical couplers which perform the function of semi-reflecting and semi-transparent separating plates usually employed in conventional interferometers. It is then possible to join the optical fiber of the reference arm and the optical fiber of the measurement arm in such manner that these fibers are subjected to the same environment in the major part of their length and that only the end section, in the vicinity of the free terminal part, of the optical fiber of the measurement arm is subjected to the phenomenon which is desired to be evaluated. The two diopters of the end faces of the optical fibers of the measurement arm and of the reference arm act as mirrors. For this purpose, they are usually metallized by any suitable conventional method.
This type of Michelson interferometer employing optical fibers is delicate to use and the quality of the measurements does not attain the theoretical performances one would expect.
Indeed, even if the measurement arm and reference arm fibers extended side-by-side through similar media, except for the terminal part of one thereof, they exhibit particular differential disturbances which are peculiar thereto and which are due to for example the ambient temperature and the mechanical stresses to which they are subjected. These disturbing phenomena merely translate the heterogeneities of the optical fibers and/or of the ambient medium. Particularly well stabilized temperatures, especially at the core of the optical fibers, must be in particular obtained. If good sensitivity is desired, calculations show that the temperature of the two optical fibers must be stabilized at least to within 0.2K and that the difference between the lengths of the optical fibers of the two arms must not exceed 3.50 mm.
These difficulties are particularly bothersome when it is desired to employ an optical fiber interferometer and, in particular, a Michelson interferometer for measuring temperatures.
As is known, the temperature of a flowing fluid it is desired to measure, and in particular of a gas in a measuring stream where the fluid is travelling at a certain velocity, is lower than the real temperature of the probe since the extraneous heating of the probe due to the flow itself is proportional to the square of the velocity of the fluid. When a fluid travelling at a relatively high velocity passes through the stream, it is found that this difference may be relatively large.
The difficulty of measuring temperature by means of a probe directly plunged into the fluid under study is therefore clear. This is why an interferometric technique is employed, consequently without physical contact with the medium to be measured which permits translating changes in temperature into variations in the index of refraction of the medium
The measurement of temperatures of a flowing fluid is required for example in particular in turbo-machines employed for producing electric power in which the fluid is steam which sometimes contains suspended droplets and in which a miniature probe must be employed in order to avoid disturbing the flow and also owing to lack of space.